A well discussed topic I know, but anyone know where I might find a replacement SG neck with an odd measurement - 30 mm to the bottom of the fretboard. It would be nice to find one, but failing that I can make a shim for it, in which case any decent SG necks to be had out there?

Doesnt the SG have set neck? Do you really want to replace the neck? The value will decrease drastically and it will cost a lot of money to replace.